Control system for electric arc furnace



Feb. 3, 1970 s. MLKAPELL 3,493,654

CONTROL SYSTEM FOR ELECTRIC ARC FURNACE Filed March 8, 1968 2Sheets-Sheet 1 w w W .3 M m 1 A m a 5 YM B mw n $20". g 92 w mm.

w 3T my J g mm 6528 92 mm ma 1| 5921013 m E p Nm On S 5528 88 6 mm B 1mm I: N J 0N T W 8 o m SE28 5% Ll J g NN Feb. 3, 1970 s. M. KAPELLCONTROL SYSTEM FOR ELECTRIC ARC FURNACE Filed March 8, 1968 r 2Sheets-Sheet. 2

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United States Patent O Int. Cl. H05b 7/18 U.S. CI. 13-12 8 ClaimsABSTRACT OF THE DISCLOSURE A control system for an electric arc furnacehaving a hearth and at least one electrode connectable to a powersupply. The control system includes regulator means for providing acontrol signal responsive to the electrical condition of the arcfurnace, electrode drive means which adjusts the axial position of theelectrode in response to the control signal, tap changer means foradjusting the voltage applied to the electrode from the power supply,and means for reducing the current flowing through the tap changermeans, immediately prior to and during a tap change, to reduce thearcing at the arcing contacts of the tap changer.

BACKGROUND OF THE INVENTION Field of the invention The invention relatesin general to electric arc furnaces, and more particularly to controlsystems for electric arc furnaces.

Description of the prior art Electric arc furnaces must have theflexibility of being able to be operated at any one of a plurality ofvoltage levels, in order to perform such functions as holding, refining,bore down, clean bottom and melt. Therefore, tap changing under loadvoltage regulating transformers are being used to regulate the primaryvoltage applied to the arc furnace transformer. The tap changermechanism may be operated in response to an operators selector switch,which the operator manually controls; or, it may be automaticallyoperated in response to computer control, or power sensitive relays.

Electric arc furnaces are often operated above their nameplate ratingsduring certain portions of the arc furnace operating cycle, such asduring the melt down period. The furnace transformer and the maincurrent carrying contacts of the underload tap changer are thus calledupon to carry short time overloads, i.e., higher than rated current, andthey are expected to do so without deleteriously affecting theirperformance, or resulting in a substantial impairment of their expectedoperating life. However, if the underload tap changer is called upon tochange taps during thew. periods of higher than rated current, thearcing contacts may be seriously stressed, burned and pitted, resultingin a short useful operating life of the arcing contacts, necessitatingcostly down-time of the arc furnace for tap changer maintenance.

SUMMARY OF THE INVENTION Briefly, the present invention is a new andimproved control system for electric arc furnaces of the type whichutilize tap changing underload, which overcomes the disadvantages ofprior art electric arc furnace apparatus and control.

More specifically, the invention is a new control system for an electricarc furnace, having a hearth, at least one electrode, and a powersupply. The control system includes regulator means for providing acontrol signal in response to the electrical condition of the arcfurnace, such as electrode current and electrode-to-hearth voltage,electrode drive means which controls the position of the electroderelative to the melt or furnace charge, in response to the controlsignal from the regulator, tap changer means for adjusting the voltageapplied to the electrode from the power supply, and means for reducingthe electrode current immediately prior to, and during the time the tapchanger means is making a tap change. The means for reducing theelectrode current, and thus the current through the tap changer, isinitiated by the signal which initiates a tap change, and includes meansfor delaying the start of the actual mechanical tap change by the tapchanger mechanism and drive means, until the electrode current and thetap changer current have been reduced. In one embodiment of theinvention, the regulator means is of the type which operates upon theprinciple of regulating the electrode current and electrode-to-hearthvoltage to a fixed or preset ratio, in response to signals responsive tothe magnitudes of the electrode current and the electrode-to-hearthvoltage. The means for reducing the electrode current reduces themagnitude of the signal responsive to the electrode-to-hearth voltage,simulating a reduction in electrode-to-hearth voltage. This causes thecontrol signal from the regulator to change the electrode position inthe direction, relative to the melt, which reduces the electrode currentto the value which will return the voltage-current ratio, as determinedby the regulator, to its preset value. When the tap change has beencompleted, the control automatically removes its temporary modificationof the electrode-to-hearth voltage responsive signal, allowing theelectric arc furnace to return to its normal operation.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages, uses andembodiments of the invention will become more apparent when consideredin view of the following detailed description and drawings, in which:

FIGURE 1 is a schematic diagram of electric arc furnace apparatus andcontrol, constructed and arranged according to the teachings of anembodiment of the invention; and

FIG. 2 is a schematic diagram of tap changer control apparatus which maybe used with the electric arc furnace apparatus and control shown inFIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, andFIG. 1 in particular, the exemplary form of the invention illustratedcomprises an electric arc furnace 10, which includes electrodes 12, 14and 16, a furnace transformer 18, a regulating transformer 20, and apower supply or source 22 of alternating potential. The position of eachof the electrodes is controlled by regulator means which is responsiveto the electrical condition of its associated electrode. For example,the position of electrode 16 is controlled by regulator means 24. Sincethe regulator means for each of the electrodes 12, 14 and 16 would besimilar, only regulator means 24 for regulating the position ofelectrode 16 is shown in FIG. 1.

Electric arc furnace 10, which is illustrated in FIG. 1 as beingthree-phase, but which may be single-phase if desired, includes acontainer or hearth 26, which is grounded at 28, and which contains acharge or melt 30. The electrodes 12, 1-4 and 16 are suspended over themelt 30, with their lower ends being in close proximity thereto.

The furnace transformer 18, in this instance, is a three-phasetransformer having a primary 32, which may be connected in Y or delta,and a secondary winding 34 which is normally connected delta, as shownin FIG. 1, or in any other desired configuration. The secondary wind- 3ing 34 is connected to electrodes 12, 14 and 16 via electricalconductors 36, 38 and 40, respectively, and the primary winding 32 isconnected to regulating transformer 20 via conductors 42, 44 and 46.

Regulating transformer 20 may be of the type shown in FIG. 1, having aY-connected excitation winding 48, tapped phase windings 50, 52 and 54,each having a plurality of taps T, numbered consecutively across thetapped winding, a delta connected tertiary (not shown) for thecirculation of third harmonic currents, and tap changer means 56, 58 and60, associated with tapped windings 50, 52 and 54, respectively. Each ofthe tap changer means 56, 58 and 60 includes tap changer drive andcontrol means, such as tap changer drive and control means 62 associatedwith tap changer means 60. Tap changer drive and control means 62 hasterminals 65, 67 and 69, which are connected to a source 64 of ACcontrol potenital, via conductors 66, 68 and 70, respectively.

Each of the tap changer means 56, 58 and 60, such as tap changer means60, includes a preventive autotransformer 72, having its outer terimnalsconnected to the tap changer mechanism 74, which includes two movablecontact arms for sequentially changing the effective tap position of thetap changer without interrupting the circuit. The center terminal of thepreventive autotransformer 72 is connected to one of the phases of thesource 22 of alternating potential. The ends of the tapped winding 54are connected to the stationary contacts of a reversing switch 76, andthe movable contact arm of thereversing switch 76 is connected toconductor 46, which is connected to primary winding 32 of the furnacetransformer 18. Thus, the voltage from AC source 22 supplied to theprimary winding 32 of arc furnace transformer 18 is modified by thetapped windings 50, 52 and 54, with the voltage across the tappedwindings either adding to, or subtracting from, the source potential,depending upon whether the position of the reversing switch connects thetapped windings in series aiding or series opposing, with respect to thesource potential 22.

It is to be understood that the tap changing underload electrical systemfor electric arc furnace shown in FIG. 1 is one of several embodimentswhich may be used. For example, the regulating transformer may beconnected in the secondary of furnace transformer 18, instead of in theprimary.

The axial positions of the electrodes 12, 14 and 16 are each controlledby a regulator. The regulator means 24 for controlling the position ofelectrode 16 relative to the melt 30, may be of any suitable type. Forexample, it maybe of the type shown in FIG. 1, wherein it obtains ameasure of the electrode-to-he'arth voltage, and a meassure of theelectrode current, and regulates the position of the electrode toprovide a predetermined ratio of current to voltage; or, the regulatormay be of the type which obtains a measure 'of the electrode current,and regulates the electrode current to predetermined values at variousportions of the electric arc furnace operating cycle.

More specifically, regulator means 24 obtains a measure of theelectrode-to-hearth voltage of electrode 16 by a potential transformer78, which has a primary winding 80 connected between the electrode 16and the grounded hearth 26, and a secondary winding 82 connected to theinput terminals 81 and 83 of a full-wave, singlephase bridge rectifier84, through an adjustable resistor 86 and an adjustable voltagecalibrating resistor 88. For purposes which will be hereinafterdescribed, resistor 88 is shorted by two serially connected normallyclosed contacts 98 and 100, which are responsive to the tap changerdrive and control means 62, as indicated generally by dotted line 102.

A resistor 90 is connected across the positive and negative outputterminals 85 and 87, respectively, of bridge rectifier 84, and theunidirectional voltage across resistor is smoothed in a wave filternetwork, comprising inductor 92 and capacitor 94. One end of capacitor94 is connected to the negative terminal 87 of bridge rectifier 84, oneend of inductor 92 is connected to the positive terminal 85 of bridgerectifier 84, and the other ends of capacitor 94 and inductor 92 areconnected together at junction 96. Thus, the magnitude of theunidirectional potential across capacitor 94 is responsive to theelectrodeto-hearth voltage of electrode 16.

Regulator means 24 obtains a measure of the electrode current flowingthrough electrode 16 by a current transformer 104 disposed in inductiverelation with conductor 40, which is connected across a resistor 106.The voltage developed across resistor 106, due to the current flowtherethrough from current transformer 104, may be transformed to ausable magnitude in a transformer 108. Transformer 108 has a primarywinding 110 connected across the resistor 106, and a secondary windingconnected through adjustable impedance means 118 to the input terminals112 and 114 of a full-wave, single-phase bridge rectifier circuit 116.The positive and negative output terminals and 122, respectively, ofbridge rectifier 116 are connected across a resistor 124, and theunidirectional potential across resistor 124 is smoothed or filtered ina capacitor 128. One end of inductor 126 is connected to the positiveterminal 120 of bridge rectifier 116, one end of capacitor 128 isconnected to the negative terminal 122 of bridge rectifier 116, andtheir remaining ends are connected together at junction 130. Thus, themagnitude of the unidirectional potential across capacitor 124 isresponsive to the electrode current of electrode 16.

The unidirectional voltages responsive to the electrode current andvoltage are compared, and a signal developed proportional to thedeviation of the electrode current and voltage from a predeterminedratio, by connecting the negative terminals 87 and 122 of bridgerectifiers 84 and 116 in common, and by connecting the stationaryportion of an adjustable resistor 132, having a movable contact arm 134,across the junctions 96 and 130.

The movable arm of resistor 106 adjusts the magnitude of the voltagedeveloped across it for a given primary electrode current. The movablearm of resistor 106 is adjusted to set the desired ratio of electrodecurrent to electrode-to-hearth voltage.

The movable arm 134 of resistor 132 is adjusted to set the magnitude ofthe output error signal, if and when the unidirectional voltagesresponsive to electrode current and voltage changes and becomeunbalanced.

Any deviation of the electrode current and voltage from thepredetermined ratio will provide a polarized error or control signalacross adjustable arm 134 and junction 96, which may be amplified inpower amplifier means 136. Power amplifier means 136 may be of anysuitable type, such as a power amplifier of the solid state, rotating ormagnetic amplifier type. The amplified control signal is applied toelectrode drive means 140 via conductors 142 and 144. Electrode drivemeans 140 controls the position of electrode 16. Electrode drive means140 may include a direct current motor 146 having an armature 148connected to conductors 142 and 144, and a field winding 150 connectedto a source 152 of unidirectional potential through an adjustableresistor 154. The armature 148 of direct current motor 146 may becoupled to a rotatable take up drum 158, which unwinds, or winds, acable 162 which is guided by pulley to lower or lift the electrode 16,according to the direction of rotation of the armature 148, and thusaccording to the polarity of the error signal. When the ratio of theelectrode current and voltage returns to the preselected ratio, thecontrol signal will drop to zero, and the drive means 140 will bedeactivated, leaving the electrode 16 in the new position untilreceiving another control signal to move the electrode to a differentoperating position.

During certain portions of the arc furnace operating cycle, the furnacetransformer 18 and regulating transformer 20 may be operated above theirnameplate ratings. This type of apparatus is designed to accommodateshort time overloads without damage. However, if the tap changerapparatus is called upon to change taps during an overload, the arcingcontacts of the tap changer mechanism may be damaged to the extent ofimpairing their useful operating life.

This invention solves this problem by using the signal tfor changingtaps to initiate a modification of the regulator circuit to reduce theelectrode current, and thus the tap changer current, by a predeterminedfixed amount. The tap change signal is also utilized to initiate a delayin the actual mechanical tap change for a short time period, which timeperiod is selected to allow the electrode current to be reduced to itsnew lower operating value. This reduced current condition is maintainedthroughout the tap change cycle, and when the tap change has beencompleted, the regulator is returned to its unmodified condition.

In the embodiment of the invention shown in FIG. 1, wherein an impedanceor balanced-beam type regulator is used, the regulator 24 may bemodified to reduce the magnitude of the current flow by simulating areduction in the electrode-to-hearth voltage. This will change theapparent ratio of current to voltage, and the regulator will act toraise the electrode and reduce the electrode current until the currentand the simulated electrode-tohearth voltage are in the predeterminedselected ratio, which changes the actual ratio.

As shown in FIG. 1, the simulated reduction in electrode-to-hearthvoltage is accomplished by connecting an adjustable resistor 86 inseries with the electrode-to-hearth voltage responsive signal, and byshorting the resistor with two serially connected normally closedcontacts 98 and 100. The opening of either contact will connect theresistor 86 to reduce the magnitude of the signal applied to the bridgerectifier 84. Thus, the unidirectional voltage across capacitor 94 willbe reduced, and a control signal will be provided between the arm 134 ofresistor 132 and junction 96, which has a polarity which will cause theelectrode 16 to be raised until the current responsive signal acrosscapacitor 128 is reduced to the magnitude where it will again providethe predetermined ratio with the voltage across capacitor 94. Theselected setting of adjustable resistor 86 will determine the magnitudeof the reduction in electrode current.

As will be hereinafter explained, contact 98 is responsive to theinitial tap changing signal, opening to initiate the reduction in theelectrode current. Delay means is utilized in the tap changer control 62to provide a predetermined short delay interval between the start of thetap change signal and the start of the actual mechanical tap change.Contact 98 remains open until the tap changer cycle is initiated, atwhich time contact 100' will open and remain open until the tap changecycle has been completed. Contact 98 closes after contact 100 opens, andcontact 100 closes after the completion of the tap change cycle, thusshorting resistor 86 after the completed tap change, returning theregulator 24 to its unmodified condition.

The tap changer drive and control means 62 for operating contacts 98 and100, and for instituting the time delay between the tap change signaland the actual mechanical tap change, is shown in block form in FIG. 1.FIG. 2 is a schematic diagram of tap changer drive and control meanswhich may be used to provide these functions.

More specifically, terminals 65, 67 and 69 of the tap changer drive andcontrol means 62 shown in FIG. 2, may be connected to a conventionalthree wire, singlephase source 64 of alternating potential, withterminal 67 being grounded at 162, and with the voltage betweenterminals 65 and 69 being substantially twice the voltage betweenterminal 65 and ground, or between terminal 69 and ground. The tapchanger drive and control means 62 will first be described as it isconventionally constructed, and then the changes required in theconventional control circuit to practice the teachings of the inventionwill be described.

Basically, as illustrated in FIG. 2, the tap changer drive and controlmeans 62 includes a tap changer drive motor 164, which drives the tapchanger means 60 shown in FIG. 1, as indicated by dotted line 166, araise relay R having an electromagnetic coil 168 and contacts R1, R2, R3and R4, a lower relay L having an electromagnetic coil 170 and contactsL1, L2, L3 and L4, a brake relay B having an electromagnetic coil 172and a contact B1, an operators tap changer control switch 174 havingnormally open contacts 175 and 177, which is usually located remotely,such as at the operators control panel, a local tap changer controlswitch 176 having normally open contacts 179 and 181, which is usuallylocated at the tap changer, a remote-local selector switch 178 having anormally closed contact 183 and a normally open contact 185, forselecting which of the tap changer control switches is to operate thetap changer, and contacts TC1, TC2 and T03 which are cam operatedcontacts responsive to the tap changer means 60 shown in FIG. 1.

The tap changer drive motor may have two windings, a lower winding 180connected between terminals 182 and -184, which, when energized, drivesthe motor shaft in a direction to connect the tap changer to a lowernumbered tap, and a raise winding 186 connected between terminals 188and 184, which when energized, drives the motor shaft in a direction toconnect the tap changer to a higher numbered tap. The lower winding 180of tap changer drive motor 164 is connected across terminals 65 and 69via contact L1 of relay L, which is a normally open contact, i.e., openwhen the electromagnetic coil 170 of relay L is deenergized. The lowerwinding 180 is also connected across terminals 65 and 69 via contact R4of relay R, which is a normally closed contact, and through contact B1of the brake relay B, which is a normally open contact.

The raise winding 186 of tap changer drive motor 164 is connected acrossterminals 65 and 69 via contact R1 of relay R, which is a normally opencontact. The raise winding 186 is also connected across terminals 65 and69 via contact L4 of relay L, which is a normally closed contact, andthrough contact B1 of the brake relay B.

The electromagnetic coil 170 of relay L is connected from terminal 67 toterminal 65 through the circuit which includes normally closed contactR3 from relay R, normally closed contact TC2 from the tap changerapparatus 60, and then either through contacts 179 and of switches 176and 178, respectively, or through contacts 175 and 183 of switches 174and 178, respectively. The contact TD1 will be assumed to be a solidconductor for the present discussion. A sealing circuit for relay L isprovided between terminals 65 and 67 via normally closed contact R3 ofrelay R, normally open contact L2 of relay L, and normally open contactTC1 from the tap changer apparatus 60.

The electromagnetic coil 168- of relay R is connected from terminal 67to terminal 65 through the circuit which includes normally closedcontact L3 from relay L, normally closed contact TC3 from the tapchanger apparatus 60, and then either through contacts 181 and 185 ofswitches 176 and 178, respectively, or through contacts 177 and 183 ofswitches 174 and 178, respectively. Contact TD1 will again be assumed tobe a solid conductor for the present discussion. A sealing circuit forrelay R is provided between terminals 65 and 67 via normally closedcontact L3 of relay L, normally open contact R2 of relay R, and normallyopen contact TC1 from the tap changer means 60.

Electromagnetic coil 172 of brake relay B is connected across terminals65 and 67 through normally open contact TC1 of the tap changer apparatus60, an adjustable resistor 190, and a diode 192. A capacitor 194 isconnected across the electrical coil 172 of brake relay B.

Contact TC1 is a normally open contact responsive to the tap changerapparatus 60. It closes once the tap changer starts to move, and remainsclosed until the tap changer completes its tap change cycle. Normallyclosed contacts TC2 and TC3 are responsive to cams on the tap changerapparatus, with contact TC2 opening when the tap changer is on itslowest numbered tap position, and with contact TC3 opening when the tapchanger is on its highest numbered tap position. Contacts TC2 and TC3are thus interlocks which prevent the tap changer from being driven pastits intended operating range.

In the operation of the tap changer drive and control means 62, theoperator actuates the remote-local selector switch 178 to select the tapchanger switch which will initiate a tap change. The local tap changerswitch is usually used only for adjustment, test and set-up purposes,with the tap changer switch located at the remote operators panel beingutilized during the actual operation of the arc furnace 10. If theremote tap changer switch -174 is selected, contact 183 will close andcontact 185 will open, as shown in FIG. 2. If the local tap changerswitch 176 is selected, contact 183 will open and contact 185 willclose. For purposes of example, it will be assumed that the remote tapchanger switch 174 is selected. Thus, contact 183 will be closed. If theoperator wishes to move the tap changer to a lower numbered tap, he willswitch the tap changer switch 174 to the lower position and will keepthe switch in this position for a short period of time, which issuflicient to allow the lower relay L to pickup and seal-in. Indicatingmeans, such as a light 196 may be connected to be energized once therelay L seals in. Thus, the operator moves the switch 174 to the lowerposition until the indicating light 196 is energized, at which time theswitch is re- .leased and it returns to its neutral position. If the tapchanger is not already on its lowest numbered tap position, contact TC2will be closed, and normally closed contact R3 of relay R will beclosed. Thus, when the operator moves swtch 174 to the lower position,closing contact 175, electromagnetic coil 170 of relay L will beenergized. When relay L is energized, contact L1 will close to energizethe lower winding 180 of the tap changer drive motor 164, contact L4will open to remove the brake, contact L3 will open to lock out theraise portion of the circuit, and contact L2 will close to seal in relayL as soon as the tap changer has moved sufficiently to close contact TC1by cam action. When contact TC1 closes, indicating light 196 will beenergized, the operator may then release switch 174, and the circuitwill be maintained through contacts R3, L2 and TC1. If the operator hadselected the local position of selector switch 178', the operation ofthe tap changer would be similar, except now contact 185 will be closed,and the operator would initiate the lower cycle of the tap changer bymomentarily closing contact 179 of the local tap changer switch 176.

If the operator wishes to change the tap changer to a higher numberedtap position, and the remote tap changer switch 174 is selected by theselector switch 178, the operator will switch tap changer switch 174 tothe raise position, and will keep the switch in this position for ashort period of time sufficient to allow the raise relay R to pickup andseal-in. Indicating means 96 may also be connected to be energized oncerelay R seals in. Thus, the operator moves switch 174 to the raiseposition until indicating light 1% comes on, at which time the switchmay be released and returned to its neutral position. If the tap changeris not already on its highest numbered tap position, contact TC3 will beclosed, and

normally closed contact L3 of relay L will be closed. Thus, when theoperator moves switch 174 to the raise position, closing contact 177,electromagnetic coil 168 of relay R will be energized. When relay R isenergized, contact R1 will close to energize the raise winding 186 ofthe tap changer drive motor 164, contact R2 will open to remove thebrake, contact R3 will open to lock out the lower portion of thecircuit, and contact R2 will close to seal in relay R as soon as the tapchanger has moved sufficiently to close contact TC1 by cam action. Whencontact TC-1 closes, indicating light 196 will be energized, theoperator may release switch 174, and the circuit will be maintainedthrough contacts L3, R2 and TC1. If the operator had selected the localposition of the selector switch 178, the operation of the tap changerwould be similar, except now contact 185 will be closed, and theoperator would initiate the raise cycle of the tap changer bymomentarily closing contact 181 of the local tap changer switch 176.

When contact TC1 closes, the electromagnetic coil 172 of the brake relayB will be energized, closing its normally open contact B1. If the lowerrelay L is energized, contact L4 will be open, and the closing ofcontact B1 has no effect on the drive motor. If the raise relay R isenergized, contact R4 will be open, and the closing of contact B1 againhas no effect on the drive motor. When brake relay B is energizedthrough the diode 192, capacitor 194 will be charged to the voltageacross the electrical coil 172. The voltage across coil 172 will bedetermined by the setting of adjustable resistor 190. When the tapchanger reaches its new operating position, contact TC1 will open andboth the relays R and L will be deenergized, and their contacts R4 andL4 will be closed. The electromagnetic coil 172 of the brake relay Bwill also be disconnected from the source 64 of alternating potential,but capacitor 194 will discharge through the electromagnetic coil 172,keeping the electrical coil 172 energized for a short period of timeafter the energized relay R, or the energized relay L, is deenergized.This time will depend upon the voltage magnitude to which capacitor 194had been charged, and is thus determined by the setting of resistor 190.Thus, contact B1 will be closed for a short period of time after contactTC1 opens, connecting both the raise and lower coils 186 and 180 of thetap changer drive motor across terminals 65 and 69, which applies abraking force to the motor, stopping the tap changer at its newlyselected tap position. After capacitor 194 has discharged to a certainvoltage magnitude, relay B will drop out, opening its contact B1, andremoving the simultaneous energization of the raise and lower windingsof the tap changer drive motor 164.

The tap changer drive and control means 62 may be modified, according tothe teachings of the invention, by adding contacts 200 and 202 to theoperators tap changer control switch 174, with contact 200 beingarranged to operate with contact 175, and contact 202 being arranged tooperate with contact 177. Or, a single contact may be used if it isarranged to close and open when either contact 175 or contact 177 isclosed and opened. Contacts 200 and 202 are connected in parallel, withone side of the parallel arrangement being connected to contact 183 ofthe remote-local selector switch 178, and the other side being connectedto terminal 67 through the electromagnetic coil 204 of a first auxiliaryrelay AI, which has a normally closed contact 98, and also through theelectromagnetic coil 206 of a time delay TD, which has a normally opencontact TD1. Contacts 175 and 177 of tap changer control switch 174,instead of being connected directly to contact 183 of selector switch178, are connected to contact 183 through contact TD1 of time delayrelay TD. The electrical coil 210 of a second auxiliary relay All,having a normally closed contact 100, is connected to be energized whencam operated contact TC1 closes. As hereinbefore described, normally.closed contacts 98 and 100 associated with relays AI and AII, areserially connected across resistor 86 in the regulator 24 shown in FIG.1.

The operation of the tap changer drive and control means, changed inaccordance with the teachings of the invention, will now be described.The operator sets the selector switch 178 to operate the tap changerfrom the operators tap changer switch 174. If the operator wishes tochange the tap changer to a lower numbered tap position, tap changerswitch 174 would be moved to the lower position, and held there, whichcloses contacts 175 and 200. The closing of contact 200 energizes theelectromagnetic coil-s 204 and 206 of relays AI and TD. When relay AI isenergized, contact 98 will open, placing resistor 86 in the regulatorcircuit, and the regulator will start to reduce the electrode currentand the tap changer current. When relay TD is energized, it will startits timed period, with its contact TD1 closing at the end of thisperiod. When contact TD1 closes, the electrode current and tap changercurrent have already been reduced by the regulator to a lower magnitude.When contact TD1 closes, the electromagnetic coil 170 of relay L will beenergized, starting the tap changer drive motor, scaling in relay Lthrough contacts TC1 and L2, and relay AII will be energized opening itscontact 100. The indicating light 196 will also be energized whencontact TC1 closes. The operator may then release the tap changer switch174 after the indicating light is energized, as the relay L will then besealed in, and contact 100 will now be open to maintain the regulator inits modified setting. When tap changer switch 174 is allowed to returnto its neutral position, relays AI and TD will be deenergized, allowingcontact 98 to close, and relay TD to reset, opening its contact T D1.

If the operator wishes to change the tap changer to a higher numberedtap position, tap changer switch 174 will be moved to the raise positionand held there, which closes contacts 177 and 202. The closing ofcontact 202 energizes the electromagnetic coils 204 and 206 of relays AIand TD. When relay AI is energized, contact 98 will open, placingresistor 86 in the regulator circuit and the regulator will start toreduce the electrode current. When relay TD is energized, it will startits timed period, with its contact TD1 closing at the end of thisperiod. When contact TD1 closes, the electrode current and the tapchanger current have already been reduced by the regulator to a lowermagnitude. When contact TD1 closes, the electromagnetic coil 168 ofrelay R will be energized, starting the tap changer drive motor, sealingin relay R through contacts TC1 and R2, and relay All will be energized,opening its contact 100. The indicating light 196 will also be energizedwhen contact TC1 closes. The operator may then release the tap changerswitch 174 after the indicating light is energized, as the raise relay Rwill then be sealed in, and contact 100 will now be open to maintain theregulator in its modified setting. When the tap changer switch 174 isallowed to return to its neutral position, relays AI and TD will bedeenergized, allowing contact 98 to close, and relay TD to reset,opening its contact TD1.

If the regulator 24 is of the type which regulates to predeterminedcurrent magnitudes at dilferent portions of the arc furnace cycle,instead of being of the impedance type shown, the electrode current maybe reduced prior to and during a tap change by connecting a resistor inthe circuit of the regulator which measures the current, and setting theregulator to provide the desired regulation with this resistor connectedtherein. Relays AI and All would each have a normally open contactconnected across this resistor, with the contacts being connected inparallel with each other. The operation of either relay would thus shortthis resistor, simulating an increase in electrode current, causing theregulator to reduce the electrode current and maintain the current atthis reduced magnitude until the tap change is completed.

In summary, there has been disclosed a new and improved control systemfor an electric arc furnace, which automatically lowers the electrodecurrent, and thus the tap changer current, when a tap change is orderedby the operator. The tap change signal immediately initiates thereduction in electrode current, while the application of the tap changesignal to the tap changer drive motor control is delayed for a shortperiod of time sufficient to allow the regulator to reduce the electrodecurrent to its new magnitude. After this delay period, the tap changesignal is applied to the tap change drive control, initiating themechanical operation of the tap changer. Thus, the arcing contacts ofthe tap changer are called upon to interrupt a lower magnitude ofcurrent than it would normally have to interrupt at a similar point inthe arc furnace cycle, which substantially increases the life of thearcing contacts. The time between maintenance periods may besubstantially extended, resulting in less furnace down time.

While the operation of the arc furnace 10 and its associated apparatushave been described relative to operator control, it is to be understoodthat the teachings of the invention may also be applied to an electricarc furnace which is under computer control, or relay control usingpower sensitive relays to initiate tap changer lower or raise tap changeoperating cycles.

Since numerous changes may be made in the above described apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit there-of, it is intended that all matter contained inthe foregoing description or shown in the accompanyingdrawings shall beinterpreted as illustrative and not in a limiting sense.

I claim as my invention: 1. A control system for an electric arc furnacehaving a hearth, and at least one electrode connectable to a powersupply, comprising:

regulator means for proving a control signal responsive to theelectrical condition of the electrode circuit,

electrode drive means connected in circuit relation with said regulatormeans, for moving the electrode in response to the control signaltherefrom,

tap changer means for adjusting the voltage applied to the electrodefrom the power supply,

and means for reducing the magnitude of the current flowing through saidtap changer means when said tap changer means is adjusting the voltageapplied to the electrode.

2. The control system of claim 1 wherein said regulator means is of thecurrent-voltage balance type, and said means for reducing the magnitudeof the current flowing through said tap changer means when said tapchanger means is adjusting the voltage applied to the electrode includesmeans for simulating in the regulator means a reduced voltage betweenthe electrode and the furnace hearth.

3. The control system of claim 1 wherein said tap changer means includesa tap changer mechanism, drive means for said tap changer mechanism, andcontrol means for actuating said drive means in response to a tap changesignal, and including delay means for delaying the start of said tapchanger drive means after said tap changer control means receives thetap change signal, until the current flowing through said tap changermeans has been reduced.

4. The control system of claim 1 wherein said regulator means is of thecurrent-voltage balance type, having a first circuit for providing asignal responsive to a measure of the electrode current, a secondcircuit for providing a signal responsive to a measure of theelectrodeto-furnace hearth voltage, and means comparing the signals ofsaid first and second circuits to provide said control signal, andwherein the means for reducing the magnitude of the current flowingthrough said tap changer means when said tap changer means is adjustingthe voltage applied to the electrode, includes means for reducing themagnitude of the signal provided by said second circuit.

5. The control system of claim 4 wherein said tap changer means includesa tap changer mechanism, drive means for said tap changer mechanism, andcontrol means for actuating said drive means in response to a tap changesignal, and including delay means for delaying the start of said tapchanger drive means after said tap changer control means receives thetap change signal, until the current flowing through said tap changermeans has been reduced.

6. The control system for an electric arc furnace having a hearth and atleast one electrode connectable to a power supply comprising:

regulator means of the current-voltage balance type providing a controlsignal responsive to the electrode current and electrode-to-furnacehearth voltage,

electrode drive means connected in circuit relation with said regulatormeans for moving the electrode in response to the control signaltherefrom,

tap changer means for adjusting the voltage applied to the electrodefrom the power supply, said tap changer means including a tap changermechanism, drive means driving the tap changer mechanism, tap changercontrol means for actuating said drive means in response to a tap changesignal, means for reducing the magnitude of the current flowing throughsaid tap changer mechanism when the control means receives a tap changesignal, and delay means for delaying the start of said tap change drivemeans after said tap changer control means receives the tap changesignal, until the current flowing through said tap changer mechanism hasbeen reduced.

7. The control system of claim 6 wherein said regulator means has afirst circuit for providing a signal responsive to a measure of theelectrode current, a second circuit for providing a signal responsive toa measure of the electrode voltage, and means comparing the signals ofsaid first and second circuits to provide said control signal, andwherein the means for reducing the magnitude of the current flowingthrough said tap changer mechanism includes means for reducing themagnitude of the signal provided by said second circuit.

8. The control system of claim 7 wherein the means for reducing themagnitude of the signal provided by the second circuit of said regulatormeans includes impedance means which is connected to reduce themagnitude of the signal provided by said second circuit from the timethe tap change signal is applied to the tap changer control means, untilsaid tap changer mechanism completes the change.

References Cited UNITED STATES PATENTS 7/1962 Ravenscroft 13-13 X 8/1965Wilson 323-435 U.S. Cl. X.R. 13-13; 32343.5

